This invention relates to protective apparatus for alternating current electrical power transmission lines, and more particularly to an improved system of protective relaying of two parallel, equal impedance transmission lines connected between otherwise independent power sources. Such improved relaying is accomplished in what may be termed the current balance or transverse differential mode, and is herein illustratively described by reference to the presently preferred embodiments thereof.
Prior to this invention, protective relaying responsive to both phase and ground faults had been practiced by a composite device manufactured, for example, by Westinghouse Electric, known as the Type HD current balance relay. That device constituted a high-speed, inductor-loop relay having double-throw contacts providing balanced current protection of an end of a pair of parallel lines using four relays, namely three phase relays and one ground relay. The system was described in the manufacturer's product manual IL41-176, dated October, 1956, superseding IL41-407C. Its inductor loop, pivoted at each end to move a switch arm, served as the secondary of a small transformer whose primary consisted of two symmetrically tapped windings connected to be energized by a derivative of respective line currents, such that with line currents flowing in the same direction, transformer action-induced loop current was proportional to the prevailing difference between such line currents. The loop was also located in a magnetic flux gap traversed by a magnetic field produced by separate current coils connected to be energized by derivatives of the respective line currents. The connections were such that with equal currents in the lines (the non-fault situation), either in the same direction or in opposite directions, no tripping tendency was produced. This is true in one case because of absence of loop current and in the other case because of absence of flux in the flux gap. However, an unbalance of line currents produced loop torque, tripping the circuit breaker in the faulted line, i.e., that having the higher current.
Use of that prior (HD) device presented serious problems, however, recognition of which led to the present invention. The chief problem, apart from the necessity of providing a relatively complex piece of equipment sensitive to calibrate and to maintain in calibration, lay in the propensity of the device to erratic performance in the presence of high-fault currents in one line or the other. In fact, on occasion, incorrect circuit breaker tripping would occur under fault current conditions, where the current transformers saturate. This is understandable from the torque equation of the HD relay loop, which is as follows: EQU T=K.sub.1 (I.sub.1 +I.sub.2)(I.sub.1 -I.sub.2)
wherein K.sub.1 is a constant and I.sub.1 and I.sub.2 are the respective transmission line currents. It will be recognized from this equation that with increasing values of line currents, the device becomes increasingly more sensitive to changes in the pervailing difference between line currents, a condition which made calibration difficult and which produced false (and erratic) operation at times due, for example, to current transformer saturation during faults. The present invention overcomes such problems. It provides a system of protection using conventional components. It presents no calibration problem requirements, either initially or to maintain calibration, especially so when components used in the system are made to readily attainable uniformity standards so as to have substantially predictable and matched electrical characteristics. This includes current transformers and conventional directional relays, components that are readily manufactured with a high degree of uniformity. It avoids erratic operation when line currents are high because it operates with essentially the same degree of sensitivity to line current differences under high fault conditions as it does under low fault conditions. This also means that it maintains equal sensitivity to faults producing relatively small fault currents as compared with faults producing relatively high fault currents.